STABLE ISOTOPE SYSTEMATICS OF SOIL WATER: IMPLICATIONS FOR CLIMATE AND UPLIFT RECORDS

An understanding of the how the isotopic signature of water changes within the soil profile is essential for interpretations of records of climate and/or orogenic uplift that depend on tree rings or authigenic minerals that inherit the soil water signature. In this study, we combined stable isotope geochemistry with climatic and soil physics monitoring to investigate how the soil water budget and styles of soil water movement vary across a climate gradient on the leeward side of the Cascade Mountains, WA. Stable isotope compositions of precipitation, snowmelt, and soil water (both mobile and total soil water) were monitored at sites along a climate gradient in central Washington State. In this transect, annual precipitation ranges from 266 cm to 23 cm and occurs mostly as snow in the winter months. For precipitation, the seasonal and annual averages of the isotopic composition become progressively depleted moving eastward (downwind) in the climate gradient as might be expected due to rainout from moist air masses. For soil water, the isotopic data indicate that a significant component of immobile soil water, which is isotopically heavy due to evaporation, resides in the shallow soil throughout the summer dry season, particularly at the driest sites. This immobile water gives rise to a total soil water oxygen isotope composition that is 3 to 5 per mil heavier than the corresponding precipitation. During the spring snowmelt, immobile soil water is largely flushed from the soil at all depths at all sites. Comparison of isotopic compositions of precipitation and soil water reveal different gradients across the transect. In general, the isotopic gradient for precipitation is steeper than that for soil water because the heavy immobile water tends to counteract the effects of precipitation rainout. However, these gradients also vary with season and with soil depth. We conclude that in order to interpret isotope records in materials that are formed in equilibrium with soil water, it is important to consider the conditions of formation including the season in which they are formed and the depth of the soil water with which they equilibrate.